If there is a rigid/semirigid rotor head like on models, then there is no problem having the center of mass above the rotor. I have sometimes found models to be more stable and perform better when flying inverted.
For soemthing like this 'bike' - the thrust vector rotates with the vehicle - it will stay lined up with the CoG. Like rockets - it may appear unstable to have the motor at the bottom, but it is actually stable.
Like on what models?
That is a bit of an exaggeration, but with modern designs, you need wind tunnel data, you have to program a simulator, so that you can test control software without crashing a zillion expensive hardware systems, you need ejection seats and an array of test pilots trained in a flight simulator that behaves exactly like the hardware is thought to behave, etc.
The advantage here is that you could program the software so that it always stays far, far away from danger zones. With fighter aircraft, that is not an option. However, if you do that, the thing will not sell.
Just wondering if it would be an interesting project to try to create a drone with two rotors instead of the current crop with four. Might be a nice playground for genetic algorithms.
Then again, of course there are already helicopters. Why do these drones need four rotors anyway? Because it is that much easier to stabilize?
The quadrotor drones are easier to control than helicopters because they don't need any control surfaces at all. You can exert a torque in any direction by adjusting the power to the rotors. Need to go forward? More power to the back, less power in front; the front of the drone dips and the thing drifts forward. Controlling the power to each engine is trivial and the dynamics are easy, so the math is pretty easy to do.
I don't know. I still think there's too much of a risk in the whole thing being top heavy due to the blades being below the rider. Envisioning being at 5,000 ft. and all of the sudden looking at the world upside down while rocketing to the ground.